1. Field of the Invention
The present invention relates to a wear resistant sintered member of which a machinability can be improved without causing decrease in strength thereof, and relates to a production method therefore. The present invention is preferably used for members, for example, valve seats of internal combustion engines which are required to have machinability as well as wear resistance.
2. Description of the Related Art
Wear resistant sintered members produced by powder metallurgy method are applied to various kinds of sliding members since desired various kinds of hard phases which cannot be produced by a typical casting method can be dispersed in a desired matrix. For example, as disclosed in Japanese Examined Patent Application Publication No. H05-055593 (hereinafter referred to as “Patent Publication 1”), 5 to 25 mass % of a hard phase consisting of 26 to 30 mass % of Mo; 7 to 9 mass % of Cr; 1.5 to 2.5 mass % of Si; and the balance of Co is dispersed in a matrix. A large number of combinations of hard phase such as the above and various kinds of matrixes have been proposed.
The wear resistant sintered alloy disclosed in the Patent Publication 1 includes expensive Co in the matrix and the hard phase. In order to meet cost performance, a wear resistant sintered alloy not including expensive Co is proposed and used in Japanese Unexamined Patent Application Publication No. H09-195012 (hereinafter referred to as “Patent Publication 2”). The hard phase in the Patent Publication 2 uses a hard phase forming powder consisting of 4.0 to 25 mass % of Cr and 0.25 to 2.4 mass % of C as an essential elements; and the balance of Fe and inevitable impurities. In the sintered alloy, the hard phase optionally includes at least one element selected from a group consisting of 0.3 to 3.0 mass % of Mo; 0.2 to 2.2 mass % of V; and 1.0 to 5.0 mass % of W. In the hard phase using the above hard phase forming powder, hard particles mainly composed of Cr carbides are precipitated in a portion of the initial hard phase forming powder, and Cr in the hard phase forming powder is diffused in the matrix. As a result, the hardenability of Fe matrix is improved, whereby the matrix is transformed to martensite. Furthermore, the hard phase has a structure of ferrite including Cr rich portion proximate to the initial hard phase forming powder. That is, the Cr carbide particles improving wear resistance are precipitated at a portion of the initial hard phase forming powder, and the Cr carbide particles are covered with Cr rich ferrite, so that removal of the Cr carbide particles is prevented. Furthermore, the Cr rich ferrite is surrounded by martensite, whereby wear resistance of the matrix is improved. A large number of wear resistant sintered alloys in combinations of hard phase in Patent Publication 2 and various kinds of matrixes have been proposed, and wear resistant sintered alloys applying the hard phase in Patent Publication 1 have been proposed.
Various hard phases have been proposed such as the above to improve wear resistance of the sintered alloy. In order to meet high efficiency of internal combustion engines in recent years, a hard phase forming alloy powder and a wear resistant sintered member using the powder are proposed in Japanese Unexamined Patent Application Publication No. 2002-356704 (hereinafter referred to as “Patent Publication 3”) and Japanese Unexamined Patent Application Publication No. 2005-154798 (hereinafter referred to as “Patent Publication 4”). Patent Publication 3 discloses an improvement of the hard phase in Patent Publication 1 and a variation of the hard phase in Patent Publication 1 in which the matrix of the hard phase is changed to Fe. In Patent Publication 3, a wear resistant hard phase forming alloy powder includes: 1.0 to 12 mass % of Si; 20 to 50 mass % of Mo; 0.5 to 5.0 mass % of Mn; and the balance of at least one element selected from the group consisting of Fe, Ni, and Co and inevitable impurities. In Patent Publication 3, the matrix includes Mn in the above manner, whereby the matrix is strengthened, the hard phase is securely adhered to the matrix, and wear resistance is improved.
Patent Publication 4 discloses improvement of the hard phase in Patent Publication 1. In Patent Publication 4, the hard phase forming alloy powder includes: 48 to 60 mass % of Mo; 3 to 12 mass % of Cr; 1 to 5 mass % of Si; and the balance of Co and inevitable impurities. In Patent Publication 4, the Mo content is increased to increase amount of precipitated Mo silicide and to form a Mo silicide group, whereby plastic flow and adhesion of the alloy are inhibited as small as possible, and wear resistance is improved.
As described above, in order to meet requirements of high output of internal combustion engines, hard phases for wear resistant sintered members have been improved, and wear resistance has been improved. Although the above wear resistant sintered members can be formed in near net shape, in some sliding members, it is necessary to machine to meet highly precise dimensions. For example, a valve seat used in an internal combustion engine is press-fitted into a head of an engine and is used. The valve seat is required to be coaxial with a valve guide which is press-fitted in the same manner as the valve seat. The valve seat and the valve guide are machined together by a tool to be coaxial with the valve guide, wherein the tool has a cutting tool integrally equipped with a cutting tool for machining the valve guide and a cutting tool for machining the valve seat. The wear resistant sintered member such as above has low machinability due to the wear resistance, and is difficult to be machined. Therefore, in order to improve the machinability of the wear resistant sintered member, various techniques for improvement of the wear resistant sintered member have been proposed and used.
As disclosed in claims 4 and 9 in Patent Publication 2, and in claim 5 in Patent Publication 3 as the most typical techniques, a powder for improving machinability, a MnS powder, or the like, is added and mixed with a raw material powder, and particles for improving machinability, MnS particles, or the like, are dispersed in pores and powder particle boundaries of the sintered alloy. Japanese Unexamined Patent Application Publication No. H04-157139 (hereinafter referred to as “Patent Publication 5”) proposes a typical technique in which at least one material for improving machinability is selected from the group consisting of a meta-magnesium silicate type mineral and an ortho-magnesium silicate type mineral, and is used together with at least one material selected from the group consisting of boron nitride and manganese sulfide. The above new materials for improving machinability have cleavage, hereby improving machinability. In Japanese Unexamined Patent Application Publication No. H04-157138 (hereinafter referred to as “Patent Publication 6”), the technique disclosed in Patent Publication 4 is applied to the alloy disclosed in the Patent Publication 1.
A different technique from the above techniques for improving machinability is proposed. In Japanese Unexamined Patent Application Publication No. 2000-064002 (hereinafter referred to as “Patent Publication 7”), the hard phase forming powder disclosed in the Patent Publication 2 is used together with at least one sulfide powder selected from the group consisting of a MoS2 powder, a WS2 powder, a FeS powder, and a CuS powder. The sulfide powder is decomposed in sintering, and Cr sulfide are precipitated as well as Cr carbides, whereby wear resistance and machinability of the hard phase are improved. In Japanese Unexamined Patent Application Publication No. 2002-332552 (hereinafter referred to as “Patent Publication 8”), a metal sulfide powder including 0.04 to 5 mass % of S is mixed with a steel powder including 0.1 to 8 mass % of Mn. The mixed powder is compacted into a green compact in a die, and the green compact is sintered at a temperature of from 900 to 1300° C., so that a sintered member is obtained. The sintered member is uniformly precipitated and dispersed with 0.15 to 10 mass % of MnS particles with particle size of 10 μm or less in grains of the overall matrix. Patent Publication 8 mentions that machinability is improved by precipitating the sulfide, the technique can be used in combination with the above technique in which the material for improving machinability is added to the raw material powder, and the machinability can be greatly improved by the above combination.
As described above, in accordance with the recent requirements, wear resistance has been improved greatly, and machinability has been improved by various techniques. However, in recent years, machinability is required to be improved more greatly, and only the above techniques for improving machinability cannot meet the present requirements. That is, in Patent Publication 8, as shown in FIG. 2, MnS is precipitated in only the Fe base alloy matrix. Therefore, the machinability is insufficient for the hard phase which becomes harder in viewpoints of improving wear resistance disclosed in Patent Publications 3 and 8.